Commuters are stranded in a train on May 30 after railway service in Tokyo was disrupted by a strong offshore earthquake that shook most of the country. (Franck Robichon / European Pressphoto Agency)

In the past two days, malfunctions in the network of sensors that detect earthquakes in Northern California have issued three false alarms, forcing the U.S. Geological Survey to make a series of embarrassing retractions.

Just after midnight Friday morning, a magnitude 6.7 quake struck off the coast of Alaska. When its waves reached sensors operated by the Northern California Seismic Network, they were mistakenly interpreted as a 5.1 temblor near the Oregon border in Lewiston, officials say.

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"There was a ghost in the machine," Don Drysdale, spokesman for the California Geological Survey, said Friday. "There was no earthquake."

The error occurred again Saturday morning after a magnitude 7.8 quake struck off the coast of Japan and the same Northern California sensors misinterpreted its wake as a 4.8 magnitude shaker near San Simeon and a 5.5 magnitude rumble near Brooktrails.

"When the waves from these big quakes hit [the Northern California] network they think it's a local quake," said John Bellini, a USGS geophysicist. "They have some kind of filtering system, but it's not working properly."

USGS officials said they are working to repair the problem and prevent future false alarms.

All three were soon removed from the USGS's website, but not before they triggered a series of news reports – including threebyThe Times -- which later had to be retracted.